1. Field
The invention relates to data compression in general and to block-based compression in particular.
2. Background
Transform coding is a common image compression technique that divides an image into sub-images or blocks for processing. Block-based compression introduces artifacts between block boundaries because blocks are independently coded. Therefore, the transform does not take into consideration the correlation between block boundaries. Accordingly, the technique typically results in low bit rate images that are heavily compressed, but contain severe compression artifacts such as blocking, ringing and motion smear.
Scalable video coding is acquiring widespread acceptance into low bit rate applications, particularly in heterogeneous networks with varying bandwidths (e.g. Internet and wireless streaming). Scalable video coding enables coded video to be transmitted as multiple layers—typically, a base layer contains the most valuable information and occupies the least bandwidth (lowest bit rate for the video) and enhancement layers offer refinements over the base layer. Most scalable video compression technologies exploit the fact that the human visual system is more forgiving of noise (due to compression) in high frequency regions of the image than the flatter, low frequency regions. Hence, the base layer predominantly contains low frequency information and high frequency information is carried in enhancement layers. When network bandwidth falls short, there is a higher probability of receiving just the base layer of the coded video (no enhancement layers). In such situations, the reconstructed video is blurred and deblocking filters may even accentuate this effect.
As a result, several post-processing algorithms for deblocking have been proposed to reduce and/or eliminate compression artifacts. However, many involve complex computations and may result in an overall blurring effect on the output image. Other de-blocking filters do not conserve edge information effectively and are typically complex to implement in hardware.